A difficult obstacle associated with the exploration and production of oil and gas is management of significant ocean currents. These currents can produce vortex-induced vibration (VIV) and/or large deflections of tubulars associated with drilling and production. VIV can cause substantial fatigue damage to the tubular or cause suspension of drilling due to increased deflections. Both helical strakes and fairings can provide sufficient VIV suppression, but can be slow and unsafe to install.
Most helical strakes consist of one or more fins that are attached to a shell, often with the fins molded into the shell. While elimination of the shell can reduce cost substantially, it is often necessary to have a shell, for example when one or more portions of the tubular need to be covered, or when marine growth protection is desired over the entire strake and tubular surface and application of marine growth protection to the tubular's outside surface is prohibited.
For many applications, the shell can be eliminated. This greatly reduces the amount of required plastic, and allows for fabrication of the strake fins at reduced cost and time.
One way to make helical strakes without a shell is to use rope or cable for the fins and wrap the rope or cable helically around the tubular. This can be fast to procure and install. However, there are obstacles associated with this method. One obstacle is that, as the tubular gets larger, the rope gets quite large and expensive. For example, the cross sectional area of a rope increases four-fold when the tubular diameter doubles. Another obstacle is that sometimes there are other adjacent tubulars around the main tubular, such as choke and kill lines for a drilling riser. If the distance between the main tubular and the closest adjacent tubular is small, it is often not possible to install the rope or cable in the space between the main tubular and the adjacent tubular.
Another way to make helical strakes without a shell is to use a segmented strake where short (relatively) rigid segments of the required fin size are strung onto a smaller diameter rope or cable which acts as the fin core. This can reduce the cost of the helical strake fins and often even decrease the fabrication time, since large diameter ropes often require custom manufacturing.
A problem associated with segmented strakes is that the individual fin segments can vibrate and rotate around the core which can potentially reduce the strake effectiveness and also reduce the fatigue life of the fin core. Another problem with simple segmented strakes is that they can still be difficult to slide in the gap between adjacent tubulars when the gap is smaller than the fin segment size. This makes it difficult, or impossible, to install the fin segments over the end of the fin core. A final problem is that, should the fin core become severed due to fatigue or any other reason, the fin segments may slide off of the fin core and into the environment (e.g. ocean).